Electrolytic cell comprising filmforming electrodes



2,290,998 ELECTROLYTIC CELL COMPRISING FILM-FORMINGELECTRODES July 28, 1942. I P. ROBINSON Original Filed July 16, 1930 Pasfon Pobu'a-Son;

attorneg Patented July 28,

UNITED STATES PATENT OFFICE ELECTROLYTIC CEIiL COMPRISING FILM- FORMING ELECTRODES Preston Robinson, North Adams, Mass., assignor to Sprague Specialties Company, North Adams, Mass., a corporation of Massachusetts Original application July 16, 1930, Serial No. 468,466. Divided and this application January 9, 1933, Serial No. 650,912.

3 Claims.

tain metals. Such metals, for instance, aluminum, when constituting the positive electrode of a cell having a suitable aqueous electrolyte, do not dissolve in the the electrolyte upon passage of the current as do most of the metals, but instead become covered with a hard film. This film oifers a high resistance to the fiow of current as long as the aluminum is the anode. However, if the aluminum is made the cathode, the film offers practically no resistance to the passage of the current. 4

Electrolytic cells of the above type are widely used as electrolytic condensers, rectifiers and lighting arresters.

Such electrolytic cells as stated comprise an aluminum anode, a suitable aqueous electrolyte;- for instance, a solution of borax and boric acidand the film is formed by subjecting the cell to gradually increasing electrical potentials, whereby the current fiow is properly regulated; the

film which consists of a partially hydrated alumi num oxide is thereby gradually formed on the surface of the aluminum, all this being well known to the art. I

When using such cells as static condensers, it is relied upon that the film once properly formed will remain unchanged and. retain its high resistance to voltages of proper polarity as long as their values do not exceed the maximum forming voltage.

This is true to a great extent when the condenser is in continuous operation under the proper voltage conditions, as in such case the film does not dissolve in the electrolyte. However, when the condenser is idle, a slow dissolution of the film in the electrolyte takes place, which is highl objectionable.

For instance, when electrolytic condensers are used in filter circuits of radio receiving sets, the periods of idleness are long compared with the periods of operation.

When after a period of idleness the condenser- In Canada June 29,

is again placed in operation, a current surge takes place due to two causes. One cause isthe loss of charge of the condenser due to leakage; this also takes place with other types of condensers, as for instance, paper impregnated condensers. However, the surge due to this cause being a transient current of very short duration, can be disregarded as a rule.

7 The second cause for a current surge is specific with electrolytic condensers and is due to the above referred to dissolution of the film. When such condensers, after a prolonged idleness are placed again in operation, the film, having been partly dissolved, offers a reduced resistance to the current flow. While this current flow gradually rebuilds the dissolved film, such rebuilding requires considerable time and if not otherwise checked, a relatively large current flow will persist for a considerable time.

The amplitude of the current will be determined by the total resistance of the circuit, which in addition to the condenser, also comprises other devices. These devices may be damaged by the excessive surge of the condenser, unless their resistance is made high enough to .prevent such. However, to provide these devices with suificient resistance is not always practicable, and even if feasible will require the use of devices of higher resistance than would be desirable for best operating characteristics of the circuit.

Not only is this current surge highly objectionable, but the repair of the film due to'sthe sudden application of the full operating voltage will be only partial, as complete repair would require a gradual slow raising of the voltage, similar to the initial forming process. In fact, after longer periods of idleness, the initial quality of the film could be only restored by repeated formation.

Thus, besides causing an objectionable surge the condensers deteriorate during idleness due to the dissolution of the film in the electrolyte, which is evidenced by a decrease in breakdown voltage and an increase in leakage current. For instance, present day condensers having a breakdown voltage of 450 volts after their formation, when left idle for a few days, show a breakdown voltage of about 425 volts only. Thus roughly stated in a few days 25 volts of film is dissolved in the electrolyte, For longer periods of idleness, dissolution of the film and reduction in breakdown voltage is correspondingly greater.

Thus the slow dissolution of the film in periods of idleness of the condenser has three drawbacks, namely, excessive current surge, lowering of breakdown voltage, and increasing of leakage current.

The object of my present invention is therefore to prevent dissolution of the film while the condenser is out of operation and thus to obtain con- However, in practice, the condenser is sub-v jected to variations of the ambient temperature and to temperature rises during its operation. The solubility of the film in the electrolyte is as a rule greater at the higher temperatures than at'the lower temperatures, with the result that the electrolyte which is saturated at a lower temperature and therefore would not attack the film,

at a higher temperature becomes unsaturated and will become saturated partly by dissolving the added solid, but partly also by dissolving the film.

By providing an excess of solid as stated and so forming the film that the electrolyte is alternately heated and cooled during the forming process, I have been able partially to take care of this objection,

However, by a further improvement, explained hereafter, I have been able to prevent altogether dissolution of the film in the electrolyte, irrespective of the duration of idleness of the condenser,

and irrespective of temperature changes during,

either operation or idleness.

Aluminum oxide and hydroxide exists in various forms of different solubility. For instance, in some forms, as ruby or corundum, aluminum hydroxide is very slow to dissolve in any solvent; on the other hand, in the form of hydrousgelatinous precipitates, it dissolves very rapidly in any good solvent. The partially hydrated aluminum oxide constituting the film, while more speedily soluble than ruby or corundum, has a much'lower rate of solubility than the gelatinous aluminum hydroxide.

By adding such gelatinous substance to the electrolyte and saturating it at a low temperature, it will remain saturated when the temperature is raised, whereby the excess aluminumoxide for saturation will be supplied by the gelatinous substance only and the film will not be attacked. In practice, I add the hydrous-gelatinous precipitate to the electrolyte, prior to the assembly of the condenser.

Or I may add to the electrolyte, a solution containing aluminum which when added will cause an excess of this gelatinous precipitate to be thIO'JIl down.

The advantage of my invention manifests itself during the original forming process of the condensers, and condensers having a pre-saturated electrolyte and provided with an excess of gelatinous precipitates show a film of greater uniformity than electrolytic condensers manufac- The most important advantage of my process,

however, is that with condensers manufactured in accordance therewith, after any period of idleness which may occur in practice, the current surge is almost negligible and the condensers retain their initial breakdown voltage and insulating resistance.

In the drawing forming part of this specification, the single figure is a partly sectionized side view of an electrolytic condenser embodying my invention.

The container it forms one-of the electrodes of the condenser, and may be of filming or nonfilming metal. The electrolyte it consists of a solution of a weak acid and a salt of a weal: acid, for instance, of an aqueous solution of borax and boric acid.

A. second electrode is of film-forming metal,

for instance, of aluminum, projects into the con- The film on the electrode is preferably formed in an electrolyte to which is added a substance which is partly soluble in the electrolyte, and which prevents dissolution of the film, for instance, a substance which has the same constituency as the film. The electrolyte is preferably saturated with this substance and preferably an excess of solid of the substance being provided. The forming electrolyte is preferably alternately heated and cooled duringthe forming process.

The electrolyte N5 of the condenser comprises a substance which is at least partly soluble in the electrolyte and which prevents dissolution of the film of the filmed electrode, such substance being for instance, of the same constituency as the film, and preferably more soluble than the film, for instance, a hydrous gelatinous precipitate of aluminum oxide with which the electrolyte is preferably saturated with an excess of the precipitate being also present in the electrolyte.

The container I0 is closed by a cover I 2 of insulating material through which projects a threaded extension I! of the electrode l3, said extension being provided with nuts l5-l5 to form one of the outside terminals of the condenser, the container I0 forming-the other terminal thereof. v

The cover 12 is provided with a vent I 8, and a peripheral gasket I9, around which the free end of the container Ill is crimped. Preferably sealing means (not shown) are also provided between the protruding end of the electrode l3 and the cover l2.

While I have described my invention in its application to electrolytic condensers, I do not wish .to be limited to such application, as my invention is equally well applicable to other electrolytic devices with film-forming electrodes;-

and while I have assumed aluminum as the material for the film-forming electrodes, other filmforming metals, for instance, tantalum, molybdenum, etc., may be substituted. Therefore I do not wish to be limited to the illustrated example of my application, but wish the appended claims to be construed as broad as permissible in view of the prior art.

What I therefore claim and desire to secure by Letters Patent is:

1. An electrolyte for an electrolytic condenser having a filmed aluminum electrode, comprising an acid and a salt and a hydrous gelatinous compound of aluminum and oxygen.

2. electrolyte for an electrolytic device having an alumium electrode provided with a film containing a partially hydrated aluminum oxide, comprising prior to use, a weak acid and the salt of a weak acid and a gelatinous compound of aluminum and oxygen.

3. An electrolyte for an electrolytic device having an electrode provided with an oxygenated film of aluminum comprising prior to use, a weak acid and the salt of a weak acid, and a substance soluble in the electrolyte more readily than said film, and comprising a gelatinous compound of 5 oxygen and aluminum. 

